This invention relates to improvements in apparatus and methods for the detection of chemical substances, and especially of halogenated compounds, such as chloropentafluorobenzene, chlorobenzene, trichloroethylene, dichloromethane, chloroform, chloroethyl ethyl sulfide, dichloro-diethyl sulfide, hydrogen chloride [HCl], bromobenzene, bromopropane, iodobenzene or iodopropane, all of which, except HCl, have a carbon-halogen chemical bond.
Many halogenated compounds, especially chlorinated hydrocarbons, are believed to be harmful to the environment and human health either through direct exposure (potential carcinogenicity) or indirectly through their adverse effect on the ultraviolet-absorbing stratospheric ozone layer. Sixteen chlorinated hydrocarbons are among the 25 organic compounds that were recently added by the Environmental Protection Agency to the list of chemicals that are to be regulated as toxic wastes under the Resource Conservation and Recovery Act. Rules have also been issued for a 50% reduction in the production and importation of chlorofluorocarbons by the year 1995.
It is an object of this invention to provide a sensor that is relatively inexpensive and portable and that can reliably and selectively detect various halogenated compounds. To achieve the required controls, it will be necessary to resort to relatively inexpensive yet well-functioning halogenated compound monitors.
Of the few sensors that have been developed for the detection of halogenated compounds, each has serious shortcomings. A chlorinated hydrocarbon gas sensor consisting of a ZnO-based semiconductor with vanadium, molybdenum and alumina catalysts was reported by M. Shiratori, M. Katsura, and T. Tsuchiya in the Proceedings of the International Meeting on Chemical Sensors, Fukuoka, Japan, T. Seiyama et al., editors (Elsevier, N.Y., 1983), pp. 119-124. Another sensor system responsive to chlorinated hydrocarbons was reported by J. Unwin and P. T. Walsh in Sensors and Actuators, 18:45 (1989). This system decomposes the chlorinated compounds over a heated platinum coil and measures the changes in the electrical conductivity of a lead phthalocyanine film that is exposed to the decomposition products. Even earlier, Stetter et al. (Sensors and Actuators, 6:269-288 (1984)) reported the detection of such compounds by room-temperature electrochemical (amperometric) sensors following exposure to a heated noble-metal filament. Commercially available photoionization detectors also exist that respond to halogen compounds. The problem with all of these sensing systems is that they are non-selective and will respond to certain halogen-free compounds, such as methane, ethanol, benzene, hexane or nitrogen dioxide. Also, many of the prior techniques lack the sensitivity that is needed. There are also gas chromatographic detectors for chlorinated hydrocarbons that are based on Hall conductivity or electron capture. However, these systems are complex, expensive, and sensitive to interferences. In addition, they can be large, not environmentally rugged, or not sufficiently sensitive (the action level for CCl.sub.4 is often 1 ppm [part per million by volume]).
It is therefore another object of this invention to provide a sensor and method that is relatively simple, inexpensive, and that can selectively detect halogenated compounds in the presence of potentially interfering substances with high sensitivity and in a rugged solid state design.
A solid-state sensor, disclosed by J. C. Loh and C. Lu in U.S. Pat. No. 3,751,968, dated Aug. 14, 1973, was claimed to be capable of detecting dichlorodifluoromethane in a concentration as low as 20 ppb (parts per billion by volume). The stone sensor was also intended for the detection of other chlorofluorocarbons, as well as of sulfur hexafluoride, chloroform, and carbon tetrachloride. This sensor is formed of a glass-ceramic mixture of sodium or lithium silicate, lanthanum oxide, and lanthanum fluoride in a preferred molar ratio of [La.sub.2 O.sub.3 ].sub.(1-2) [LaF.sub.3 ].sub.(3-4.5) Na.sub.2 SiO.sub.3. The preparation of this sensor involves the formation of a surface depletion layer through application of "a biasing D.C. voltage of 1-10 volts" at a temperature above 500.degree. C., preferably 600.degree. C., for about 24-48 hours. No information is disclosed in the patent about the performance of this sensor. However, in view of the wide range of preferred molar ratios, one would expect the performance to vary widely from unit to unit.
U.S. Pat. No. 5,226,301, dated Jul. 13, 1993, a sensor is disclosed by J. R. Stetter and Z. Cao that bears some resemblance to that of U.S. Pat. No. 3,751,968 but overcomes the above-cited disadvantages of the earlier sensor. The newer sensor consists of a bead of sodium lanthanum fluoride silicate, having the molecular formula NaLa.sub.4 (SiO.sub.4).sub.3 F, as determined by x-ray (diffraction) crystallography, in which are embedded two noble metal electrodes, preferably a straight platinum wire near the center and a helical platinum wire near the periphery of the bead. A current passing through the helical wire maintains the sensor temperature at about 550.degree. C. by resistive heating. The electrical resistance between the two wires is deduced from measurements of the current passing through a fixed external resistor mounted in series with one electrode when a substantially constant voltage, typically about 3 volts, is applied between the wires. An increase in the measured current (at constant voltage) is an indication of the presence of a halogenated compound in the sample of air to which the bead is exposed. The sensor is preferably controlled by a micro-processor or microcomputer that also performs data processing. This system not only detects a halogenated compound of interest, but also measures its concentration. The lifetime is limited by the history of exposure. Continuous exposure to chlorinated vapors (ca: 10 ppm) can limit lifetime. Also, performance may be limited for applications where fast response and long lifetime are needed.
It is therefore still another object of this invention to provide a sensor having an extended lifetime and that has well-defined, improved, and reproducible performance characteristics.
The lifetime limitation appears to arise from a build-up of an insulating substance between two sensor electrodes. A similar build-up may adversely affect the performance and lifetime of other solid-state sensors. It is therefore an object of this invention to provide apparatus and methods for improving the performance and lifetime of any solid-state sensors that may be adversely affected by a build-up of an objectionable chemical reaction product resulting from operation and exposure of the sensor to a monitored analyte.
Other objects of the invention will become obvious to professionals in various fields, such as industrial hygiene, medical monitoring, process control, military, aerospace, or pollution monitoring, following perusal of the complete specification.